Temperature compensated r. c. network



March 1, 1960 H. E. RUEHLEMANN 2,927,224

TEMPERATURE COMPENSATED R C .NETWORK Filed Aug. 25, 1955 H. E. RUEHLEMANBY d ATTOR 13's United States Patent() TEMPERATURE COMPENSATED R.C.NETWORK Herbert E. Ruehlemann, Allentown, Pa., assignor to the UnitedStates of America as represented by the Secretary of the NavyApplication August 25, 1955, Serial No. 530,650

13 Claims. (Cl. 307-109) i (Granted under Title 35, U.S. Code (1952),sec. 266) Work which compensates forresistance variations therein causedby temperature fluctuations.

In electric timing devices such as electric f ,uzes for example using RCtime constant circuits, the time is some function of the time constantof the resistance and capacitance of the RC circuit. Any variation inthe value of resistance or capacitance due to temperature changes causesa corresponding uctuation of time. There are capacitors commerciallyavailable which have a negligible temperature coetlicient of capacitancein the range of 60 F. to +l75 F., but, at this time, no resistors havingnegligible temperature coefficient of resistance are available forutilization in electric time fuzes.

Heretofore, the problem of resistance changes due to temperaturevariations has been solved by employing carbon deposit resistors whichhave a negative temperature coeicient in series with wire woundresistors which have a positive temperature coefficient. These resistorsare arranged in such combination that anr increase in resistance of one,caused by a fluctuation in temperature, will be exactly counterbalancedby the decrease of resistance of the other resistor due to this sameiluctuation in temperature. However, wire wound resistors of themagnitude of several megohms are too large and cumbersome forutilization in ordnance devices like fuzes. Furthermore, the best carbondeposit resistors, when of small size, still have a temperaturecoefficient of resistanceof -300 parts per million, and most of themhave a temperature coetlicient in the range of -900 to -1200 parts permillion; and, when carbon resistors are used in combination with wirewound resistors, the temperature coefficient range of the carbonresistors is such as to cause excessive time deviations due totemperature changes to be effectively employed in precision time deviceslike time fuzes for bombs, mines, or missiles.

The general purpose of this invention is to provide a time constant RCnetwork which overcomes the above disadvantages. In accordance with theinvention, there is provided a pair of equally charged capacitors of thenegligible temperature coeicient type and a pair of carbon depositresistors arranged to present a discharge path for the capacitors, theresistors being of the negative temperature coefficient type and havingdifferent temperature coefficients of resistance. The pair of resistorsare so arranged with respect to the pair of capacitors that the resistorwith the lesser temperature coeflicient solely provides the dischargepath for one of the capacitors, the discharge path for the othercapacitor being provided by both resistors.

It is an object of this invention to provide a new and improvedresistance-capacitance time constant network for electric timingcircuits.

With the foregoing in mind, it is an important object of the presentinvention to provide a time constant network in which the time constantthereof is not appreciably affected by temperature variations.

Another object of the invention is to provide a time constant networkwhich compensates for time fluctuations caused by temperature variationsto which the time constant network may be subjected.

A further object is to provide a time constant network having asubstantially constant time constant.

An essential object of the invention is to provide, in a time constantnetwork, a resistive discharge circuit f of non-uniform temperaturecoeicient of resistance for energy storage means, whereby any tendencyof the storage means to deviate from its normal rate of discharge due totemperature variations is compensated by the non- 1 uniform temperaturecoeliicient characteristic of the discharge path.

A significant object of the invention is to compensate for timedeviations introduced in a resistance-capacitance time constant networkby temperature variations by providing a pair of resistive dischargepaths of different temperature coefiicients of resistance. Anotherimportant object of the invention is to provide, as the resistivedischarge circuit for a resistancecapacitance time constant network, apair of resistors of the negative temperature coefficient of resistancetype and of different temperature coeliicients of resistance, wherebyany deviatory tendency of the time constant of the network due totemperature changes is compensated by regulatory resistive coaction ofthe pair of resistors.

A still further important object is to provide a temperature compensatedtime constant network having a pair of capacitors of the negligibletemperature coehcient of capacitance type and a pair of resistors ofdifferent negative temperature coefficients of resistance so arrangedwith respect to the capacitors that the rate of discharge of one of thecapacitors through both the resistors regulates the discharge rate ofthe other capacitor, whereby time deviations in the network due totemperature changes are compensated.

A more specific object of the present invention is to provide atemperature compensated time constant network having a pair ofcapacitors of the negligible temperature coeiiicient of capacitance typeinitially charged in parallel and a circuit selectively adapted topresent simultaneously a pair of discharge paths to the capacitors, oneof the discharge paths being formed by a single resistor of the negativetemperature coeicient of resistance tvpe and the other discharge pathbeing formed by the aforesaid single resistor and a second resistorhaving a greater negative temperature coetiicient of resistance.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood bvreference to the following detailed description when considered inconnection with the accompanying drawings in which like referencenumerals designate like parts throughout the figures thereof andwherein:

Fig. l illustrates a conventional and simplified time delay circuit; and

Fig. 2 is the circuit arrangement, partly in schematic and partly inblock diagram, of a timing circuit incorporating the time constantresistance-capacitance network of the invention.

Referring now to the drawings, there is shown in Fig. l, whichillustrates a well known time constant network a capacitor C which isinitially charged to a value E0 from a source indicated as a batterythrough switch arm S and its stationary contact a. When capacitor C`Patented Mar. 1,` 1960 apanage where E is the initial voltage on thecapacitor, Ec is the voltage across the capacitor C at any time t, andRC is the time constant T of the circuit.

If the rated value of resistor R is varied an incremental amount Ar due.to a change in temperature, the new time constant, assuming that thechangeY in capacitance is negligible, is:

T=(R:Ar)c (2) and the time is accordingly changed by an incrementalamountk At, resulting in the expression From Equation 3, it isy readilyapparent'that, in thearrangement of Fig. l, the time will deviatedirectly proportional to the change in resistance caused by temperaturefluctuations.

Referring now to Fig. 2, there is shown a timing circuit incorporatingthe time constant resistance-capacitance network of the presentinvention. The time constant networkof the invention, shown in solidlines, consists of a pair of capacitors C1 and C2 initially connected inparallel. and in thev same. polarity through lead 8, switch arm S',contact a', conductor 9 and through conductors and 11. Initially, thecapacitors C1 and C2, which are of. the negligible temperature coeicientof capacitance type, are equally charged to a value E0 from any suitablesource such as a battery having its negative terminal connected throughcontact a' and stationarywiper arm to the negative plates of capacitorsC1 and C2 and its positive terminal connected through contact a andswitch arm S to the positive plates of capacitors C1 and C2.

It; is tobe understood that, if the timing circuit of Fig. 2; isemployed in an electric fuze of an explosive missile, the contacts a andd are xedly disposed in the breech of the missile firing mechanism so asto engage wiper arm S and stationary wiper arm 15, respectively, uponinsertion of the missile into the breech of the firing mechanism and thevoltage source E0 is connected to contactsy a and d through conductors13 and 14, respectively, and is mounted on a suitable supportingstructureassociated with the tiring mechanism whereby the capacitors C1and C2 are charged upon insertion of the missile into thetiringrnechanism breech. Switch arms S and S', whichY are shown as.ganged for movementl in unison, may be of the inertia type switcheswhich are thrown to their b and b positions, respectively, by set-backforce as the missile is'propelled by the tiring mechanism.

Although the invention is described herein as being used in electricfuzes for explosive missiles and is ofspecic utility therein, it is tobe understood that the inventionl is not limited to such use but also isof utility in any type of electric timing device wherein precisiontiming is desirable, and in such cases the switches S and S may be ofany type suitable for the purpose and' need not necessarily be ganged.

A resistor R1, of the negative temperature coetiicient of resistancetype, has one end thereof connected to the negative. plates of.capacitors. C1 and C2A andthe other' end thereof connected to xedcontact b'. A second resistor R2, having a greater negative temperaturecoeicient of resistance than resistor R1, is serially connected betweenthe capacitors C1 and C2 and is initially shorted by switch S andconductor 9 during the time capacitors C1 and C2 are being charged.Resistors R1 and R2 may be of the boron carbon type or of the carbondeposit type. Also, in accordance with the teachings of the invention,the negative temperature coefcient of resistance of resistor R2 ymust begreater than that of resistor R1. For example, the temperaturecoefficient of resistor R2 may be -900 parts per million and that ofresistor R1 may be -300 parts per million. These values are only by wayof illustration, and the invention is not limited thereto.

The time actuated system 5, which may consist of one of many knownconventional electrically actuated circuits controlled by a timeconstant network, is connected to contact b. through lead 6 and to thenegative plates of capacitors C1 and C2 through conductor 7. UponswitchesS and S being thrown to their respective b and b' positions, thetime actuated system 5 is connected across capacitors C1 and C2 which,at the instant switches S and S' are thrown to the b and b positions,start to discharge through resistors R1 and R2 and continue to Ydischarge therethrough. until a predetermined interval of time haselapsed, as determined by the selected values of resistors R1 and R2and. capacitors vC1 and 2, whereupon the magnitude of potentials oncapacitors C1 and C2 have been reduced to such values as to enableactuation of. time. actuated system 5 in its prescribed manner such, forexample, as igniting a detonator or energizing a relay or solenoid. Itis torbe noted that the ohmic values of resistors R1 and R2 and thecapacitive values of capacitors C1 and C2 may be of any desired valuesselected to produce a predetermined time delay, as is well known tothose skilled in the art, and have no. bearing on the concept of theinvention.

In the operation of the invention as to compensating for time constantvariations due to temperature fluctuations andassuming that capacitorsC1 and C2 have been equally charged to the value E0, movement ofswitches S and S', either by set-back force in the case of a missilefuze orr by suitable meansv either manual or automatic, in the case ofconventional electric timing devices, initiates the timing cycle of thetime constant network, and capacitor C1 starts to discharge throughresistor R1, producing a. current flow i1 which results in an IR valueacross resistor R1 determined by the. values of i1 and resistor R1.However, capacitor C2, which initially was charged to the same potentialas C1, can discharge only when the potential across C1 becomes lowerthan the potential on C2. As soon as capacitor C1 has reduced to apotential below that of capacitor C2, capacitor C2 discharges throughresistors R2 and R1 in series with a current i2. This current i2 throughresistor R1 causes an additional IR drop across resistor R1 whichincreases the voltage drop across resistor R1 and thereby delays thedischarge of capacitor C1. Assuming that the time constant network is atsuch temperature that resistors R1 andl R2 are at their rated values,the capacitors C1 and C2 continue to discharge in this manner throughresistors R1 and R2 at the rate determined by the rated time constant ofthe network.

On the other hand, if the temperature increases, the resistance valuesof resistors R1 and R2 will decrease incrementally in proportion totheir temperature coeicients of' resistance. Accordingly, since theresistive value of R1 has decreased, capacitor C1 will start todischarge through. resistor R1 at a faster rate than its normal rate ofdischarge. But, since the temperature coethcient of resistor R2 isgreater than that of resistor R1, the resistance of resistor R2 hasdecreased incrementally more than has resistor R1, and, as aconsequence, capacitor C2 will also discharge more rapidly than. its`normal discharge rate; resultingiiran increase iri magnituieof currenti3. This increased current i2 appearing across resistor R1 in'turnincreases the IR drop across resistor R1, thusA proportionallyV delayingthe discharge of capacitorCl and therebycounterbalancing the tendency ofcapacitor C1 to discharge faster due to temperature effectsovnlrlesistorlRl. In this manner, the time constant network of theinvention compensates for resistive variations therein introduced bytemperature fluctuations. In the case of a decreasing temperature, thecapacitors C1 and C2 function in a converse manner from that describedabove to attain the same result.

It has been found that excellent temperature compensation in a mediumtime range is obtained when the resistor R2 has a negative temperaturecoeflicient of resistance twice, or thrice, that of resistor R1. Formedium and long time range, complete temperature compensation isobtained by employing a resistor R2 having a negative temperaturecoefficient of resistance which is 3.6 times greater than that ofresistor R1.

The teachings of the invention are not limited solely to the utilizationof negative temperature coeiiicient resistors, but, if desired, theinvention can be practiced with positive temperature coefiicientresistors. If positive temperature coefficient resistors are employed asresistors R1 and R2 in the time constant network of Fig. 2, resistor R2must have a greater positive temperature coefiicient of resistance thanresistor R1 in order to obtain temperature compensation as taughtherein.

Briefiy stated in summary, the invention contemplates to compensate fortime variations introduced in time constant networks by temperaturefluctuations by providing a pair of resistors of different temperaturecoefficients to form the discharge p-aths for a pair of capacitors, theresistors being of either both negative or both positive temperaturecoeicient types with the resistor having the lesser temperaturecoefficient solely presenting a discharge path to one of the capacitorsand both resistors in series presenting a discharge path to the othercapacitor.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that, within the scope of the teachings herein and theappended claims, the invention may be practiced otherwise than asspecifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. A temperature compensated time constant network comprising energystorage means adapted to exponentially discharge a predeterminedpotential, a resistive circuit of non-uniform temperature coefiicient ofresistance adapted to present a discharge circuit to said storage meansand to form therewith a network of predetermined time constant, andselective circuit means coupling said storage means and said resistivecircuit to form sa-id network of predetermined time constant wherebysaid storage means discharge said predetermined potential through saidresistive circuit, the non-uniform temperature coefficientcharacteristic of said resistive circuit compensating for any timefluctuations introduced in said predetermined time constant bytemperature variations.

2. A time constant network which inherently compensates for timefluctuations introduced by temperature variations comprising energystorage means adapted to exponentially discharge a predeterminedpotential,

a pair of resistive paths of different temperature coefficients ofresistance, and selective circuit means for discharging said storagemeans through said resistive aths.

p 3. A time constant network which inherently compensates for timefluctuations introduced by temperature variations comprising an energystorage network having a pair of discharging branches, a pair of re- 6,sistors of differen'ttemperaturecoefficients of resistance, andswitching means for serially interconnecting said resistors across oneof said branches and for connecting the lresistor with thelessertemperature` coefiicient across the other of said branches.

4. The network of claim 3 wherein each of said pair of resistors has anegative temperature coefiicient vof resistance.

5. The network of claim 4 wherein said discharging branches includecapacitors which inherently have negligible temperature coefficients.

6. The network of claim 4 wherein said pair of resistors are of thepositive temperature coefficient of resistance type.

7. A time constant network which inherently compensates for timefiuctuations` introduced by temperature variations, comprising a pair ofcapacitors adapted to be equally charged from a source of energy, a pairof resistors of different temperature coeficient of resistance adaptedto provide a discharge circuit for said capacitors, and circuit meansincluding a switch for connecting said resistors to said capacitors toprovide a discharge circuit therefor, said resistors being so connectedwith respect to said capacitors that the rate of discharge of one ofsaid capacitorsl through both said resistors regulates the dischargerate of the other capacitor whereby time deviations in the network dueto temperature changes are compensated.

8. The network of claim 7 wherein said resistors have negativetemperature coefficients of resistance and said capacitors have anegligible temperature coefficient of capacitance.

9. A time constant network which inherently compensates for timefluctuations introduced by temperature variations, comprising a pair ofcapacitors of negligible temperature coefficient of capacitance, adischarge path for each of said capacitors, a first resistor ofpredetermined temperature coefficient of resistance defining thedischarge path for one of said capacitors, and a second resistor inseries with said first resistor forming the discharge path for the othercapacitor whereby the discharge of said other capacitor through saidfirst and second resistors regulates the discharge of said one capacitorthrough said first resistor to thereby compensate for time fluctuationscaused by temperature variations, the temperature coefiicient of saidsecond resistor being greater than said predetermined temperaturecoefficient of resistance.

10. The network of claim 9, wherein the temperature coefficient ofresistance of said second resistor is 3.6 times greater than saidpredetermined temperature coeficient of resistance.

ll. The network of claim 10, wherein said first and second resistorshave negative temperature coefficients of resistance.

l2. In a resistance-capacitance time constant network, means forcompensating for time fluctuations caused by resistance changes due totemperature variations and comprising a first resistive discharge pathadapted to have a first potential exponentially discharged thereacross,and a second resistive discharge path adapted to have a second potentialequal to said first potential exponentially discharged thereacross, saidsecond path having a greater temperature coefiicient of resistance thansaid first path and said second path including said first path as aportion thereof whereby time uctuations caused by resistance changes insaid resistive paths due to temperature variations are compensated bycommutual resistive coaction of said paths.

13. A time constant network which inherently compensates for timefluctuations introduced by temperature variations, comprising a pair ofcapacitors having a negligble temperature coefiicient of resistance,circuit means for equally charging said capacitors in parallel, andselective circuit means for discharging said capacitors through a pairof discharge paths, one of said paths being formed by a single resistorof predetermined negative temperature coeicient of resistance connectedacross one of said capacitors, the other of said paths being formed by asecond resistor serially connected with said single resistor across theother capacitor, said second resistor having a negative temperaturecoecient of resistance greater than Said predetermined temperaturecoeicient.

References Cited in the file of this patent NITED STATES PATENTSRuhlemann 1211119, 19,32 Stansbury Dec. 17, 1935 Haynes Sept. 5, 1944Southeimer Oct. 26, 1948

